The Treasury was last night forced to deny that Alistair Darling was drawing up secret plans to raise VAT to 18.5% in the next parliament in an attempt to fill the black hole in the government's finances created by the recession and the massive stimulus package announced on Monday.

Officials insisted that a document outlining an intention to raise VAT to a record rate had been put on a website by mistake, and the chancellor had rejected the idea before Monday's pre-budget report.

Details of the proposed change - which would infuriate retailers and intensify the political row about tax - emerged in online background notes provided by the Treasury to explain the temporary VAT reduction announced in the PBR. The web document reads: "The proposed changes will reduce this [the VAT rate] to 15% from December 1 2008 until the end of 2009. The standard rate will then return to 17.5% from January 1 2010, and subsequently increase to 18.5% in 2011-12."

A hard copy of the notes was issued with the PBR and signed off by Stephen Timms, the financial secretary to the Treasury, on Monday, the day of the PBR. It is identical - including Timms' signature - except the planned increase in VAT to 18.5% had been removed. The online version was removed last night.

The plan to increase VAT was withdrawn from the Treasury document on Friday, a government source said. This is likely to be seized on by the Tories as a sign that the proposal was under serious consideration just days before the PBR.

The Treasury and Revenue & Customs had slightly different versions of the story last night. A Treasury spokesman said: "Someone appears to have emailed documents to a website that should not have been sent. There is no plan to increase VAT. The chancellor said in the PBR that he wanted to raise revenue in the fairest way possible and by targeting those that have done best in recent years."

Revenue & Customs, however, said that the VAT increase had been a fully worked-up option that had been ditched at the last moment in favour of the 45% tax rate on those earning more than £150,000.

One tax specialist said last night: "Why did they reject it? Is it because they don't want to tell us about it, or because they really don't want to do it?"

The Treasury's own estimates show the new tax on the rich will raise only £1.5bn - less than a third of the £5bn generated by a 1% hike in VAT. The Institute for Fiscal Studies (IFS) said earlier yesterday that the 45% income tax band might raise no extra revenue because higher earners will find ways of avoiding paying more tax.

Evidence that a VAT rise had been under consideration was seized on by David Cameron. He said: "They said no hidden manifestos, everything above board, show you how we will pay for our policies. And now we know they are planning this extra tax rise. That is why the budget doesn't add up, that is why there is such a big black hole, that is why everyone is saying this budget isn't convincing. There is a secret tax bombshell coming down the road at every family in the country."

The document emerged at the end of a day which saw the Conservatives granted an emergency debate on the PBR and the IFS warn that the credit crunch had "punched" a permanent £60bn hole in the economy which would force the government to raise taxes or cut spending.

The Tories scored a rare political victory when the Speaker, Michael Martin, granted their request for a Commons debate on the PBR on the basis that it represented one of the most significant economic statements since the second world war. There is normally a debate after the budget.

The shadow chancellor, George Osborne, told the speaker: "This was not just a report, but an emergency budget and a reckless gamble with public finances. "

Last night the first opinion poll since the PBR showed a Conservative lead of just four points. The YouGov survey for the Daily Telegraph put the Tories on 40% (down two points on a similar poll last month), Labour on 36% (up three) and Liberal Democrats on 14% (down one).